Digital image sensors typically provide, both in video and in still imaging, a duration of time known as an exposure, or integration, period, over which pixels accumulate an electronic signal that is in turn related to the photon signal impinging on that pixel during the integration, or exposure, period.
In many conventional digital image sensors, the nth row has an exposure time that lasts from t_n to t_n+t_integration, while the n+1th row has an exposure time that lasts from t_n+1=t_n+t_row until t_n+1+t_integration. Here, t_row is the time to read and reset a given row.
This is known as a rolling shutter, wherein the positions of time of the start and end of the integration period are different for different rows. In general, higher-numbered rows acquire images later in time than earlier rows.
Rolling shutter leads to artefacts during imaging. For example, if a square object moves across the screen, its square shape is distorted to a trapezoid as a consequence of the rolling shutter delays.